The use of a single programmed digital microcontroller in a disk drive to supervise a high level digital interface to a host and also to supervise a digital head positioner servo loop is known. One example is found in commonly assigned U.S. Pat. No. 5,005,089, entitled "High Performance, High Capacity MicroWinchester Disk Drive", the disclosure of which is hereby incorporated by reference. In the disk drive described in the referenced '089 patent, a polyphase optical encoder included a scale attached to a voice coil rotary actuator and provided two relative position phases P1 and P2 which were digitized and used by the microcontroller to determine head position. The position was further refined by a calibration routine which measured and stored reference head position data from the data storage surfaces. Also present in the '089 patent disk drive was a high level, digital interface, such as a SCSI interface (or an AT bus level interface). The microcontroller in the '089 patent disk drive divided its time between the head position servo and the interface, so that the head was correctly positioned as needed during seeking and track following, and so that host commands were received, decoded and acted upon, and command cleanup was performed. Since the servo tasks during track seeking required more frequent attention of the microcontroller, the time division between servo functions and data transfer (interface) functions was dynamically altered. The disk drive described in the referenced '089 patent was subsequently improved as described in commonly assigned U.S. Pat. No. 5,227,930, and entitled "Head Position Recalibration for Disk Drive", the disclosure of which is hereby incorporated by reference. The disk drive described in this later application provided for increased concentric data track densities, in part, by sampling storage disk runout and deriving and following an average track centerline based upon multiple circumferential samples.
The need to store higher quantities of data continues. Higher data densities may be obtained by increasing the number of data tracks per unit measure, such as per inch, abbreviated "TPI". Higher data densities may also be obtained by increasing the data transfer rates, and by adjusting data transfer rates to radial position of the data transducer head, as relative head-disk velocity varies with disk radius.
In order to increase the number of data tracks, the head gap may be made narrower, and disk runout characteristics may be followed. As shown in e.g. U.S. Pat. No. 4,530,020, it is known to provide a reference track adjacent to the outer diameter containing centerline information which is collected, digitized and stored as a digital record. The record is then accessed in synchronism with disk rotation to provide an error correction signal for correcting repeating spindle runout errors. In order to provide the regular periodic correction, the disk drive microcontroller must carry out servo service routines at regular intervals synchronized to disk rotation.
Similarly, in disk drives employing embedded servo sectors for head positioning, the disk drive microcontroller is interrupted at each servo sector and thereupon devotes its processing capability to determining present head position and developing any positional correction signal that may be required. An example of such a disk drive is found in commonly assigned U.S. Pat. No. 4,669,004, entitled "High Capacity Disk File with Embedded Sector Servo", the disclosure of which is incorporated by reference.
In the disk drive examples provided hereinabove, the need for regular periodic attention of the embedded microcontroller to service the head position servo loop has resulted in delays in performing other important tasks, such as host command overhead, host command cleanup, and cache memory management, for example. These delays have slowed data transfer operations. This drive performance drawback is overcome by the present invention.